This invention relates to a four directional (4D) material and method for reinforcing a conical shaped object with fiber elements and to a conical preform formed from fiber elements, wherein the fiber volume fraction along the axis and circumference of the conical surface remains invariant.
In certain applications wherein an object is expected to be exposed to a relatively harsh environment, typically a composite material is used to form the object or to be applied to surfaces of the object for protection against the environment and/or for reinforcing the object. It is desirable that the composite material have a substantially constant fiber element reinforcement fraction over the surface of the object so that significant composite property disparities between areas of the surface are avoided, thereby permitting accurate predictions of composite material response to the environment. It has been especially difficult to obtain a constant fiber reinforcement fraction along the axis of a conical or other axially increasing diameter shell structure. Further, the resulting reinforcement material should not exhibit discontinuities or a seam along the join line or other portion of the conical surface.
Prior three-dimensional fiber reinforcement patterns have drawbacks when configured to form or conform to a conical surface. These include failing to maintain constant radial and in-plane (i.e. over the conical surface) fiber reinforcement fractions along the length of the conical surface while maintaing continuous paths for winding the in-plane fibers through a radially disposed fiber array; or failing to provide continuous paths for winding the in-plane portion of the fiber reinforcement material while maintaining a lower variation of radial and in-plane fiber volume fractions along the length of the conical surface. For the former case, which is typical of three-directional polar reinforcement designs, significant variations in structural properties occur along the length of the conical surface since the radial and in-plane fiber reinforcement fractions vary with axial position along the conical surface. In the latter case, discontinuities in the in-plane fiber reinforcement paths result in structural deficiencies and make fabrication of a fiber reinforcement preform impractical.
U.S. Pat. No. 4,519,290--Inman et al discloses a three-dimensional 4D braided preform fabrication for making annular or conical sections to be used in producing articles. The 4D fiber architecture includes a plurality of rods of carbon fibers uniformly distributed over the surface and inserted into a conical mandrel perpendicular to the conical centerline as shown in FIG. 2 of the patent. Oblique carbon or graphite fibers are then passed alternately over and under similar longitudinal fibers around the radially extending rods to provide a triaxial braided pattern having a repeating unit cell that is illustrated in FIG. 6 of the patent. However, the 4D fiber architecture described in U.S. Pat. No. 4,519,290 does not achieve invariance of fiber volume fraction along the conical surface.
Another 4D configuration is described in U.S. Pat. No. 4,400,421 --Stover, wherein the four directions of groups of reinforcing fibers remain parallel to repeating elements of the group. Although a similar unit cell to that employed in the present invention is obtained, a method for deploying or conformally mapping a planar array onto a conical surface to obtain constant fiber volume without discontinuities or a seam at the join line is not described or illustrated.
A 4D triangular fiber arrangement is described in a DTIC report ADBO49350 entitled "Boron Nitride--Boron Nitride Composite Material" by Potter and Place. FIG. 4 of the Potter and Place report illustrates a cylindrical configuration having three triangularly related fibers disposed in a plane perpendicular to the axis of the cylinder and one fiber disposed in a plane parallel to the axis of the cylinder. This fiber arrangement would not generate a constant fiber volume fraction of radial fibers over a conical shell, nor would a constant fiber volume fraction be obtained in the conical surface direction of the shell without addition of new fiber ends.
U.S. Pat. No. 4,570,166--Kuhn et al, describes conformal mapping of a planar sector of a circle, having a grid pattern of isosceles triangles inscribed therein, onto the surface of a cone corresponding to the sector in the context of an RF transparent conically shaped antenna shield structure. The vertices of the triangles are used to situate RF components in the antenna shield structure.
Accordingly, it is an object of the present invention to provide a method for forming a three dimensional fibrous element preform for a conical object, wherein the preform includes an invariant fiber volume fraction along the axis and circumferential direction of the object.
Another object is to provide a material fabricated from fiber elements that may be configured in a conical shape and have an invariant fiber volume fraction along the axis of the conical shape.
Yet another object is to provide a method for reinforcing an object having a conical surface, wherein a single fiber element may be used to form the in-plane fraction while obtaining invariant fiber fraction along the axis.
Still another object is to provide a conical shaped preform or material for reinforcing a conical surface wherein there is no seam along the join line or other area of the conical portion and further wherein discontinuities throughout the conical portion are avoided.